In recent years, projectors are used in halls, movie theaters, and the likes. A dual-lamp projector is used when a single-lamp projector can not achieve required luminance for a large screen in a hall, a movie theater, or the like (e.g. Patent Literature 1).
A dual-lamp projector includes two high-pressure discharge lamps as light sources, and lighting devices which are provided for the high-pressure discharge lamps respectively, and each of which turns on the corresponding high-pressure discharge lamp. Each lighting device includes an alternating-current generator and a controller. The alternating-current generator generates an alternating current having a rectangular waveform to be supplied to the high-pressure discharge lamp, based on an input control signal. The controller controls the alternating-current generator by inputting control signals to the alternating-current generator. Each controller has an internal clock, and generates the control signals based on the internal clock.
In such a dual-lamp projector, either one of the following two methods has been conventionally used for controlling the alternating currents to be supplied to the two high-pressure discharge lamps. One is a synchronous control for synchronizing the two alternating currents by adjusting the phases thereof to conform to each other, and the other is an asynchronous control which does not synchronize the phases.
Generally, a dual-lamp projector is configured to equalize the periods of the alternating currents. However, when the asynchronous control which does not synchronize the phases is used, the alternating currents do not have a same period in some cases, for the following reasons.
In each of the controllers in the projector, the period of the internal clock sometimes changes according to the operating environment and the likes. The change amount of the period of the internal clock is not always the same for each controller. Thus, with the asynchronous control, despite an attempt to equalize the two alternating currents, the periods of the alternating currents to be generated will be different when the change amounts of the periods of the internal clocks have a difference.